There are many systems on the market that try to provide four wheel drive capability. Those systems are all designed to engage all four wheels but also allow a speed differential across the axle. However, many of those systems do not provide true four wheel drive where each wheel provides substantially the same speed during all drive conditions. Instead, the systems permit some degree of slippage.
Current Four Wheel Drive Bi-Directional Overrunning Clutch Systems
FIG. 1 illustrates the drive system for a conventional four wheel drive vehicle with a front bi-directional over-running clutch. The drive system includes four wheels. The rear left wheel RLW is connected to a rear differential RD through a rear left axle RLA. The right rear wheel RRW is connected to the rear differential RD through a rear right axle RRA. The front left wheel FLW is connected to a front differential FD through a front left axle FLA. The front right wheel FRW is connected to the front differential FD through a front right axle FRA.
The rear differential RD is connected to the transmission T through a rear drive shaft RDS. The front differential FD is connected to the transmission T through a front drive shaft EDS.
Straight Line Operation:
During straight line driving while the vehicle is in a four wheel on demand mode (i.e., four wheel drive engages only when needed) both rear wheels RLW, RRW are the primary drive wheels and are connected through the rear differential RD to rotate at the same speed. In a non-slip condition of the rear wheels, the front drive shaft FDS is engaged to the front differential FD, but the front axles FLA, FRA are not engaged with the front differential. That is, the front axles FLA, FRA and front wheels FLW, FRW are generally in an overrun condition such that the front differential FD is not driving the front axles FLA, FRA and, therefore, not transmitting any torque to the front wheels. This means that the front wheels FLW. FRW are free to rotate at their actual ground speeds.
In order for the front wheels to be engaged, the rear wheels must slip (break traction) or spin increase speed approximately 20% faster than the front wheels. While driving in a straight line, once the rear wheels slip 20%, the overrunning condition in the front differential ED is overcome and both front axles are engaged. This results in the transmission T transmitting torque to the front wheels thru the front drive which is geared in a way that decreases the vehicles ground speed. When the ground speed has increased so as to cause the rear wheel speed to be rotating less than 20% faster than the ground speed, or the speed of the rear wheels has decreased so as to be rotating less than 20% faster than the ground speed, the front wheels will start to overrun again and no torque will be transmitted to the front wheels.
Turning Operation:
In a corner all four wheels are trying to rotate at different speeds. This is shown on the chart in FIG. 4 which depicts wheel revolutions vs. turning radius for all four wheels. For a vehicle with a locked rear axle or solid axle (i.e., an axle where the rear axles RLA, RRA are connected, either physically or through gearing, such that they always rotate at the same speed) the ground speed is dictated by the rear outside wheel due to vehicle dynamics (i.e., the rear outside wheel has to cover more circumferential distance than the rear inside wheel when turning around a common axis.) Since both rear wheels are rotating at the same speed and the rear outside wheel is the drive wheel the rear inside wheel is beginning to scrub or drag on the ground. This can cause inefficiencies, turf wear and/or tire wear.
The primary reason conventional bi-directional overrunning clutch four wheel drive systems have a 20% under drive is for turning. With the rear outside wheel dictating ground speed the front inside wheel will go slower than the rear outside wheel as shown in FIG. 4. If there is no under drive the bi-directional overrunning clutch for the front inside axle would engage and begin to drive torque. This would cause the front inside wheel to travel at an incorrect speed and would create inefficiencies, turf wear, tire wear and, more importantly, torque steer.
As mentioned above, during a turn the rear outside wheel is dictating ground speed, the rear inside wheel is scrubbing or dragging, and the front wheels are overrunning. Referring to FIG. 5 which depicts the percentage difference between the front and rear wheel speeds versus the turning radius of a locked rear axle, once the rear outside wheel slips or spins a certain percentage, dictated by vehicle geometry and turning radius, the bi-directional overrunning clutch controlling the transfer of torque to the front inside wheel will engage and drive torque through the front inside wheel. At this time both rear wheels and the front inside wheel are driving torque and their speed is dictated by the drive line, not ground speed. The front outside wheel is still overrunning allowing it to spin at the rotational speed dictated by ground speed and vehicle geometry. When both rear wheels and the front inside wheel slip a certain percentage, again dictated by vehicle geometry and the turning radius, the bidirectional clutch controlling torque transfer to the front outside wheel will engage and torque will be transmitted to all four wheels, even though three of the wheels would be slipping.
Wedging
The existing drive system is prone to a condition called wedging. Wedging occurs when torque is being driven through the bidirectional over-running clutch and a rapid direction change occurs. This can cause the rollers in the clutch to be positioned or locked on the wrong side of the clutch profile preventing the output hubs from overrunning. The effect causes the front drive to act like a solid axle, but with the 20% speed difference in the drive line it results in scrubbing of the front tires. This condition can cause excessive tire wear and turf wear. This also effects steering effort and stability of the vehicle. The vehicle will try to maintain a straight line due to the effect of the front drive acting like a solid axle.
Because of the wedging condition in the current systems precautions are put into place to help reduce wedging. One of these precautions is the use of a cut-off switch so that when the vehicle is shifted from the forward direction to the reverse direction so as to automatically disengage the bi-directional overrunning clutch (for example, shutting off the coil that is indexing the roll cage). This system also uses the cut-off switch when transitioning from the reverse direction to the forward direction. Another way to reduce wedging is the use of a switch, when the brakes are applied, that will interrupt power to the 4 wheel drive system. Many other methods can be used to reduce wedging, but none are 100% percent effective with the 20% difference in drive line speeds.
Conventional Drive Systems:
A common conventional drive system would have the same vehicle layout as in FIG. 1, but the mechanisms in the front and rear differentials would be different. Most common drive systems have an open differential with the ability to be locked into a solid axle in both the front and rear differentials. The drive line in a conventional system would also be using a drive line that is geared to a 1:1 ratio.
Straight Line Operation:
During straight line driving while the vehicle is in four wheel drive and all the axles are unlocked, all four wheels are rotating at the same speed. This is due to the drive line being geared at 1:1 ratio and the front and rear differentials are being driven at the same speed and no differentiation is needed across the axles. This is also the case when any or both of the front and rear differentials are in a locked position creating a solid axle.
Turning Operation:
Conventional four wheel drive systems will normally have the rear differential locked and the front drive will be in the open state until the solid axle mode is selected by the user. During turning with a solid axle in the rear differential and an open differential in the front, only one tire is turning at the correct ground speed. Due to vehicle dynamics the rear outside wheel is considered the drive wheel and is turning at ground speed. The inside rear wheel is being driven at the same speed as the rear outside, but the ground speed is slower. This causes the inside rear wheel to scrub or slip during a turn.
Since the two front wheels are connected to an open differential, they are allowed to differentiate across the axle. However, the differential is being driven at an incorrect speed. That is, the front open differential takes the input speed and averages it across the axle. In a normal non slip condition the average speed across the axle is centered about the middle of the vehicle. Since the rear outside wheel is traveling at a different speed (or arc) than the average of the two front wheels, both front wheels are scrubbing when in a turn causing un-needed drive line torque or drive line bind.
Once the operator selects the solid axle mode of the vehicle, both front wheels are locked together and they now rotate at the same speed. When turning, the outside front wheel is going slower than what ground speed dictates, thus causing the wheel to scrub. At the same time the inside front wheel is going faster than the ground speed dictates causing it to, likewise, scrub.
Due to the wheels being driven at the wrong speeds in a corner, conventional drive systems are not very efficient. They cause severe turf damage or wear due to the tires scrubbing. They also cause tire wear due to the scrubbing. The tires being driven at the wrong speeds also cause issues with steering and turning performance of the vehicle. The difference between ground and actual wheel speed results in the wheels trying to straighten the vehicle out. This cause's increased wear in steering components, as well as rider fatigue since increased input is needed to maintain the vehicle in the turn. Many manufacturers have added power steering to try to minimize operator input when cornering because of the four wheel drive operations.
A need therefore exists for an improved four wheel drive system that incorporates bi-directional overrunning clutches in a drive system that minimizes scrubbing in all wheels while permitting 1.1 or near 1:1 gear ratio between the front and rear axles.